Conventionally, in a construction machine and the like, an axial hydraulic piston pump driven by an engine and an axial hydraulic piston motor driven by pressure oil are widely used.
For example, the axial hydraulic piston pump is provided so as to integrally rotate with a rotational axis rotatably provided in a case and has a cylinder block in which a plurality of cylinders elongating in an axial direction are formed so as to be spaced apart in a circumferential direction, a plurality of pistons each of which is slidably inserted into each cylinder of the cylinder block to move in the axial direction in association with rotation of the cylinder block to suck and discharge operating oil, and a valve plate provided between the case and an end face of the cylinder block in which a suction port and a discharge port communicating with each cylinder are formed. Then, in the hydraulic pump, when a drive shaft rotate-drives, the cylinder block rotates together with an operating shaft in the case, the piston reciprocates in each cylinder of the cylinder block and the operating oil sucked from the suction port into the cylinder is pressurized by the piston and is discharged to the discharge port as the pressure oil.
Herein, when a cylinder port of each cylinder communicates with the suction port of the valve plate, a suction process in which the piston moves in a direction to protrude from the cylinder from a start point to an end point of the suction port to suck the operating oil from the suction port into the cylinder is performed. On the other hand, when the cylinder port of each cylinder communicates with the discharge port, a discharge process in which the piston moves in a direction to approach in the cylinder from a start point to an end point of the discharge port to discharge the operating oil in the cylinder to the discharge port is performed. Then, by rotating the cylinder block so as to repeat the suction process and the discharge process, the operating oil sucked from the suction port into the cylinder in the suction process is pressurized and discharged to the discharge port in the discharge process.    Patent Document 1: Japanese Laid-Open Patent Application Publication No. H07-189887    Patent Document 2: Japanese Laid-Open Patent Application Publication No. H08-144941